There also exists a large and
interesting literature devoted to eigenvalues inside a spectral gap
of a self-adjoint operator, with massive Dirac-Coulomb Hamiltonians as prime
examples [1]}, [2]}, [3]}. Massless Dirac-Coulomb Hamiltonians do not have a gap, so
strictly speaking this question does not appear in our paper. Nevertheless, we
believe that methods of our paper are relevant for the eigenvalue
problem in the massive case.
